At work, I unfortunately only have a laptop and one monitor, so I usually keep my Outlook on the laptop and code on the external screen.
At home, however, I use two monitors (the third one is non-functional at the moment :( ).
For Web Development I use one monitor for writing code and the other one for viewing the result (+debugging) and doing google searches.
For Game Programming I usually fill up all screens with source code, since I manage my code in many files and several projects. When hitting the debug button, I usually run the program on one screen and debug on another.
So, yeah, I think it boosts productivity as you can view several things next to each other (easier for comparisons etc.) and you don't have to tab through a lot of windows (for me, at least, it's quite a lot of windows all the time)